Especially in the field of medicine, image fusion methods are being used for better diagnosis of a possible pathological state. The purpose of image fusion is to combine and visually overlay medical images or image series of a patient, which contain supplementary information to one another. The method most commonly used at present for visually overlaying image series or images is so-called alpha blending. In this case, the gray values of the two images or image series are weighted with a blending factor (alpha value) and the weighted average value of the two images to be fusioned is represented for each image point. By varying the alpha value, it is possible to vary the relative weighting of the images or image series in relation to one another. Discrimination of the two fusioned images or image series in the fusion image output on a monitor is possible, in particular, whenever the respective relevant data of two image series contain anatomically separable image ranges, for example the skull cap in a CT picture combined with the brain tissue in an MR picture.
If the data are not anatomically separable, however, then the image data are “mixed” by the alpha blending. On the one hand, the information about which of the two image series the individual pixels to be represented belong to is lost by the mixing. But since the images or image series to be fusioned are often complementary, and therefore contain different information to be interpreted, information about the membership of individual image points or image ranges is very important. Furthermore, a significant contrast reduction occurs for each pixel of the fusion image owing to the blending of the two original gray values. This means that the alpha blending generates a relatively low-contrast—compared with a separate representation of the two images or image series to be fusioned—visualization of the two images or image series to be fusioned. The contrast loss due to alpha blending is a problem for interpretation of the image data in the fusion image. Difficulties arise, in particular, whenever images that have been recorded in the same mode are to be fusioned, for example two CT pictures which show the same anatomical area. As a rule, the gray-value distributions in these pictures are very similar, or approximately equal. Sensible alpha blending, which leads to a meaningful fusion image, cannot be generated here.
One known way of avoiding the problem of contrast reduction caused by the mixing of gray values in alpha blending consists in using a so-called “threshold mode” for blending of the two images or image series over one another. In this case, only image data of the first image or of the first image series which lie in a particular predeterminable gray-value range are visualized first. The image data of the second image series in another predeterminable gray-value range which is disjoint from the first gray-value range, that is to say it differs significantly therefrom, are then drawn over the already represented image data of the first image series, without blending, that is to say without mixing. The boundary between the two gray-value ranges is set by a so-called “threshold value”. The alpha blending is hence replaced in the threshold mode by an overlay of image data, although the image ranges to be overlaid must necessarily lie in disjoint gray-value ranges.
This recording of disjoint gray-value ranges, which is a prerequisite for using the threshold mode with a single threshold value, however, is not feasible whenever two images or image series to be fusioned are similar to one another in terms of their gray-value distribution. This is the case, in particular, whenever the two image series to be fusioned have been generated by the same modality, that is to say using the same recording instrument.
Such “intramodality fusions”, that is to say fusions of images recorded with the same instrument, are often carried out for procedural monitoring (for example to monitor tumor growth or to monitor the success of a medication). In this case, the relevant images of the image series to be fusioned lie in the same anatomical range and in the same gray-value range. The threshold mode is therefore equally unusable here (no disjoint gray-value ranges) as alpha blending is (contrast reduction and mixing of the gray values, which the observer can then no longer assign to one of the two images or image series).
From “FOLEY, J. D. et al.: Computer Graphics-Principles and Practice, Addison-Wesley, 1996, pages 835–834” it is known to provide different regions of an image with different alpha values. Basically, standard alpha blending in the scope of image fusion is described therein. DE 199 49 877 A1 describes the principle of 3-D visualization by volume rendering. This primarily involves determining a transfer function for the volume rendering algorithm. The volume rendering principle does not relate to fusioned image data or to alpha blending. Lastly, U.S. Pat. No. 5,042,077 A describes the nonlinear windowing of an image.